There is a need for an efficient, simple and economical means of communication between controllers, sensors, and controlled devices in distributed systems. The types of systems which would benefit greatly from such a communications system include, but are not limited to, distributed energy systems such as heating and air conditioning, other process systems, distributed alarm systems for fire and intrusion, and many others. The requirements for such a communication system are economy, for obvious reasons, simplicity of interconnection, to make the system useful for a wide range of applications, and efficiency of message arbitration and addressing, to allow use with low bandwidth media.
There have been several different local communications links developed in recent years. Some of these local networks provide one-way communications from a single, fixed terminal. These links, for example, the one marketed by BSR, Inc., are adaptable to a variety of applications.
Another form of local network uses a single data path and an arbitration scheme which depends upon detection of collisions and re-transmission with a random time delay. This type of system is typified by ETHERNET, presently marketed by Xerox Corporation.
Another form of local network avoids most collisions by assigning each port a time slot, which rotates through all terminals in the network. Such networks are known as "token" networks, because the time-slot assignment is mediated by passing around a "token", or special symbol. This type of network is typified by CHAOSNET.
Yet another form of local network is the IEEE STD-488 instrumentation bus, which uses a master controller to handle all communications.
The subject invention is intended to fill a need which is not quite met by any of the above mentioned local networks. It provides two-way communications with low cost and high utilization of the communications medium.